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How do scales, feathers, and hair develop from vertebrate skin?

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A pangolin walks through grass with a blue circle around it and the word endangered written in white across the image. — Integument The pangolin is the only mammal in the world covered in scales.

In vertebrate biology, scales, feathers, and hair are specialized integumentary appendages that develop through complex interactions between the epidermis and the underlying dermis.1

Scales

Scales are primarily dermal derivatives, though their structure varies significantly across species. In many fishes, scales form within the dermis as skeletal elements, such as bony plates or denticles. These may be composed of dentine and enamel, or they may be thin, overlapping bony plates. In terrestrial vertebrates, the term "scale" often refers to the hardened, keratinized surface layer of the epidermis, which is shed periodically.213

Feathers and Hair

Feathers and hair develop from similar embryonic rudiments involving a local thickening of the epidermis and an accumulation of mesenchyme cells in the dermis.3

Hair: Development begins as an epidermal thickening that grows downward into the dermis to form a follicle, which invests a small stud of dermal tissue called the dermal papilla. The hair shaft grows outward from this root as a cylinder of compacted, keratinized cells.34
Feathers: These develop when the epidermal thickening bulges outward to create a hollow, fingerlike protrusion with a connective tissue core. This structure subsequently branches to form the characteristic barbs and barbules of the feather.3

In both cases, the mature structures consist of dead, keratinized cells.3